This invention relates to a plural-line terminating apparatus and a method of OAM processing in this apparatus. More particularly, the invention relates to a plural-line terminating apparatus for accommodating a plurality of lines, converting signals from these plurality of lines to ATM cells and sending the ATM cells to an ATM switch, sending ATM cells from the switch upon converting the cells to line signals, and executing OAM processing of a plurality of lines. The invention further relates to a method of executing this OAM processing.
Asynchronous Transfer Mode (ATM), which is the core of a broadband ISDN, makes it possible to divide a variety of information such as voice, video and data into fixed-length packets, referred to as cells, and to transfer the information over a network at high speed while handling the information in a consolidated manner. Monitoring network failures in the operation of an ATM network is important, and use is made of OAM (Operation, Administration and Maintenance) Fault Management cells (AIS cells, RDI cells, etc.) to monitor connection failure.
FIG. 19 is a diagram for describing the basic mechanism of OAM (Operation, Administration and Maintenance) flow. Shown in FIG. 19 are an end point 1 on a transmitting side, an end point 2 on a receiving side, and connecting points 3a, 3b, 3c . . . at which OAM cells can be extracted/inserted. If a defect A has been sensed at a certain connecting point 3b, an AIS (Alarm Indication Signal) cell is sent in the downstream direction in order to notify the downstream side of the connection of the defect A. Upon receiving the AIS cell or directly detecting a defect B, the end point 2 downstream of the connection sends back an RDI (Remote Defect Indication) cell to the opposing end point on the upstream side. As a result, the end point 1 upstream of the connection is capable of managing defects on the transmitting and receiving sides in both directions, and the connecting points 3a.about.3c are capable of discriminating the occurrence of defects and the locations thereof by monitoring the AIS and RDI cells.
FIGS. 20A and 20B are formats of OAM cells, in which FIG. 20A shows the format (F4 OAM flow) of an OAM cell for a VP connection and FIG. 20B the format (F5 OAM flow) of an OAM cell for a VC connection.
The F4 OAM flow implements (1) discrimination and notification of faults in a VP connection (a VP-connection fault management function), and (2) notification of error rate, cell loss rate and cell mixing rate of user information cells (a VP-connection performance management function). The F5 OAM flow implements a VC-connection fault management function and a VC-connection performance management function in a manner similar to that of the F4 flow.
The OAM cells of the F4 and F5 OAM flows both consist of a 5-octet ATM cell header 100 and a 48-octet ATM cell payload 101. The ATM cell payload 101 is composed of the following information:
(1) 4-bit OAM cell type 101a; PA1 (2) 4-bit function type 101b; PA1 (3) a 45-octet function specific field 101c, which specifies fault category and fault location; PA1 (4) a 6-bit reserved field 101d; and PA1 (5) a 10-bit ECC field 101e. PA1 fault management type (Fault Management) 0001; PA1 performance management type (Performance Management) 0010; and PA1 activation/deactivation management type Activation/Deactivation) 1000.
FIG. 21 is a correspondence table of OAM cell type 101a vs. function type 101b. The following are advised as the OAM cell types:
The fault management type 0001 includes (1) an alarm indication signal (AIS), which is a warning of fault detection, (2) remote defect indication (RDI), (3) continuity check and (4) loopback. The performance management type 0010 includes (1) forward monitoring and (2) backward reporting. The activation/deactivation management type includes (1) performance monitoring and (2) continuity check.
Since an OAM cell (FIG. 20A) for a VP connection traverses a path the same as that of a user cell which flows through the VP connection, the OAM cell has a VPI number the same as that of the user cell, and specific VCI values (VCI=3, VCI=4) so that the cell will be identified as an OAM cell. VCI=3 identifies the cell as being a segment OAM cell, and VCI=4 identifies the cell as being an end-to-end OAM cell. A segment OAM cell is a cell inserted/extracted in a segment and is valid only in a segment interval; it is not transmitted outside of a segment interval. An end-to-end OAM cell is a cell valid end to end of a set connection; it is discarded at the end point of a connection.
Since an OAM cell (FIG. 20B) for a VC connection traverses a path the same as that of a user cell which flows through the VC connection, the OAM cell has a VPI value and a VCI value the same as those of the user cell, and specific payload type identifiers (PTI=100,PTI=101) so that the cell will be identified as an OAM cell. PTI=100 identifies the cell as being a segment OAM cell, and PTI=101 identifies the cell as being an end-to-end OAM cell.
Thus, in a case where an AIS cell has arrived at a certain ATM connection point, the connection point undergoes a transition to an alarm state at reception of the AIS cell. Restoration to the normal state is made in response to non-reception of the AIS cell for 2.5.+-.0.5 seconds or reception of a user cell (alarm-status recovery cell). Since the function-specific field 101c of the Fault Management cell indicates the details of a fault (fault category and location, etc.), as mentioned above, at the time of a failure the maintenance personnel may operate the network by referring to the detailed information that has been recorded in the function-specific field 101c of the Fault Management cell.
FIG. 22 is a block diagram illustrating the configuration of an ATM switching system. Shown in FIG. 22 are subscriber interfaces (or line IFs) 11.sub.11.about.11.sub.1n, 11.sub.21.about.11.sub.2n, 11.sub.31.about.11.sub.3n, 11.sub.41.about.11.sub.4n connected to corresponding lines (transmission lines), multiplexer/demultiplexers 12.sub.1.about.12.sub.4, an ATM switch unit 13, a system controller 14 and a maintenance terminal 15. The ATM switch unit 13 is connected to the plurality of multiplexer/demultiplexers 12.sub.1.about.12.sub.4, switches input cells from certain multiplexer/demultiplexers and outputs the cells to prescribed multiplexer/demultiplexers. The multiplexer/demultiplexers 12.sub.1.about.12.sub.4, which are connected to the pluralities of line interfaces 11.sub.11.about.11.sub.1n, 11.sub.21.about.11.sub.2n, 11.sub.31.about.11.sub.3n, 11.sub.41.about.11.sub.4n, respectively, multiplex incoming cells from a plurality of line interfaces IF and output the cells to the ATM switch unit 13. Furthermore, the multiplexer/demultiplexers 12.sub.1.about.12.sub.4 demultiplex and output incoming cells, which arrive from the ATM switch unit 13, to the pertinent line interfaces.
The line interfaces 11.sub.11.about.11.sub.4n, which are connected to the corresponding multiplexer/demultiplexers 12.sub.1.about.12.sub.4, each extract an ATM cell from the payload of a frame signal of a prescribed format (e.g. a SONET frame) that has entered from the line, convert the cell to one having the cell format within the switch and output the cell to the multiplexer/demultiplexer. The cell format within the switch is provided with information TAG for routing purposes. The ATM switch unit switches a cell to a prescribed path by referring to this tag information TAG.
Furthermore, the line interfaces 11.sub.11.about.11.sub.4n convert the cells of the switch cell format that enter from the multiplexer/demultiplexers 12.sub.1.about.12.sub.4 to cells having the ATM cell format, map each ATM cell to the payload of the SONET frame and send the ATM cell to the line side. The system controller 14 controls the line interfaces 11.sub.11.about.11.sub.4n, multiplexer/demultiplexers 12.sub.1.about.12.sub.4 and ATM switch unit 13.
FIG. 23 is a block diagram showing the construction of a line interface. One line interface 11 is provided in correspondence with one set of outgoing/incoming lines and has a one-line/one-package construction in which one line interface 11 is formed in one package. The multiplexer/demultiplexer (MUX/DMUX) and ATM switch are shown at 12 and 13, respectively.
The line interface 11 includes a physical terminator 21 for forming a frame signal having a prescribed format that has entered from the outgoing line into a cell stream and outputting the cell stream to the side of the ATM switch, and for forming a cell stream, which has entered from the side of the ATM switch, into a frame signal having the above-mentioned format and sending the frame signal to the corresponding line. A UPC (Usage Parameter Control) processor 22 performs monitoring to determine whether the reported value of transmission capacity and the actual cell inflow quantity conform. When cells in excess of the reported value flow in, processing for discarding cells in contravention of the stipulation is executed. The line interface 11 further includes a billing/NDC processor 23, which performs billing control and NDC control for counting the number of passing cells and creating billing data, and an OAM processor 24.
In a case where a line (transmission line) is constituted by e.g., an optical cable, the physical terminator 21 has an optoelectric converter for converting a light signal to an electric signal, an electro-optic converter for converting an electric signal to a light signal, and a SONET terminator. The SONET terminator deletes overhead (section overhead SOH and path overhead POH) from a frame signal having a SONET STC-3C (156 Mbps) format shown for example in FIG. 24, extracts ATM cells from the payload field PL and then converts the format to the cell format within the switch and outputs the cells. Further, the physical terminator 21 sends a cell stream, which has entered from the switch side, to the line upon converting the cell stream to a frame having the SONET STS-3C format shown in FIG. 24.
Thus, the conventional line interface has the one-line/one-package architecture. Consequently, the OAM processor 24 constituting the line interface 11 is so adapted that it executes OAM processing without being aware of the identification of the line. In other words, the OAM processor 24 executes OAM processing without being aware of the particular line via which a cell has entered and without being aware of the particular line to which a cell is output.
Accommodating a plurality of lines in a single package by use of large-scale integration and controlling the plurality of lines of the package by a single OAM processor is now in demand. In order to satisfy this demand, it is required that identification of a connection, which is possible in the prior art based solely on the VPI/VCI, include line identification as well. More specifically, it is required that the connection be identified by a VPI, VCI and line identifier. As a consequence, a connection management table that is necessary for OAM processing becomes larger in size and there is an increase in the amount of hardware.
In addition, to control a plurality of lines by a single OAM processor, it is required that OAM processing be executed upon distinguishing among the individual lines. Moreover, it is necessary to so arrange it that user cells will not be discarded owing to the band of the line being exceeded due to insertion of OAM cells.
In order to manage a line between adjacent ATM switches, there are cases where a network VP connection (NVP connection) is established for the line and line management processing is executed by sending and receiving OAM cells for the NVP connection between the adjacent ATM switches. Such an OAM cell for the NVP connection is terminated by the OAM processor of the first of the adjacent ATM switches and is not sent downstream. Therefore, in a case where the OAM cell for the NVP connection notifies of a line failure, a problem which arises is that the line failure will not be communicated to ATM switches other than these ATM switches, namely to ATM switches located downstream.
Further, in order to execute OAM processing regarding an NVP connection, it is required that a management table for the NVP connection be provided in addition to the management table for the ordinary VP/VC connections. Thus the number of management tables increases. Consequently, in a case where a plurality of lines are controlled by a single OAM processor, the capacity of the management tables increases and so does the amount of hardware.